Heat-treatment of irregular metallic objects



A g- 1951 H. F. KlNCAlD ET AL HEAT TREATMENT OF IRREGULAR METALLIC OBJECTS Filed July 9, 1947 Med/270110 Aug. 21, 1 51 H. F. KINCAID ET AL HEAT TREATMENT OF IRREGULAR METALLIC OBJECTS Filed July 9, 1947 3 Sheets-Sheet 2 W24, f z mff 1951 H. F. KINCAID ET AL 2,564,906

HEAT TREATMENT OF IRREGULAR METALLIC OBJECTS Filed July 9, 1947 5 Sheets-Sheet 5 j Parr Patented Aug. 21, 1951 UNITED STATES PATENT OFFICE HEAT-TREATMEN T OF IRREGULAR METALLIC OBJECTS Application July 9, 1947, Serial No. 759,888

10 Claims.

Our invention relates to heat treatment and has particular relation to the heat treatment of irregular metallic objects for the purpose of producing a hard surface or skin on the objects.

Irregular metallic objects, such as gears which are included in the transmission of an automotive vehicle, crank shafts and similar items are subjected to repeated mechanical impacts. It is desirable that such objects be provided with a layer of hard metal in the region where the impacts are to be applied. Such hardened surfaces are produced by heat treatment.

One method of heat treatment in accordance with the prior art, of which we are aware, is called carburizing. In the carburizing process the metallic object to be treated is immersed in a carburizing medium and its entire surface is subjected to the action of chemical agents. A thin hardened layer is produced over the whole surface of the object. The chemical agents to which the object is subjected are costly; they are consumed at a high time rate during the treatment and require repeated replacement. The carburizing process is time consuming. This process is therefore not suitable for the production of hardened objects at the high rate and the low cost demanded by modern mass production of automotive vehicles.

In accordance with other teachings of the prior art, of which we are aware, the objects are hardened, not only on the surface, but throughout in the region where the impact is applied. For example, gears are so heat treated that the teeth are hardened throughout, the hardening extending a short distance below the roots of the teeth. The hardening is produced by heating a gear in the region to 'be hardened (the teeth and the roots) to a temperature of approximately 1600 F. and thereafter quenching the heated portion of the gears.

The difficulties associated with, and disadvantages of, this process will be understood from a consideration of the process as applied to a gear. Because of a high temperature differential which exists during the heating operation between the heated region and the unheated body of the gear, high tensile stresses are produced between the two sections of the gear. When the gear is quenche the stresses are further modified. The expansion of the teeth which arises by reason of the chemical changes which constitute the hardening process further modifies the stresses. We have found that the stresses produced during this heat treating process cause defects to develop in gears so treated. The gears 2 are frequently cracked in the region of the teeth and the roots, and the teeth do not have the wearing properties which are essential for such components.

It is accordingly an object of our invention to develop a method of heat treatment which overcomes the above difiiculties and shall be inexpensive and highly efficient;

Another-object of our invention isto develop a method of heat treatment which shall be peculiarly suitable-for the supply of components, such as gears, racks, crank shafts and similar items to satisfy the quality, quantity and cost demands of modern mass production of automotive vehicles.

A further object of our invention is to provide a method of heat treatment for irregular metallic objects, said method involving the contour or zone hardening of said. objects with the use of induction current of different electrical frequencies.

A still further object of our invention is to provide an electrical heat treating process for producing hardening-equivalent to that produced by chemical processes, such as the carburizing process. I

It is still another object to provide an improved method for heat treating articles with the utilization ofan electrical induced current, said treating operation leaving a minimum of unfavorable residual stresses in the article after hardening.

Another object is to provide an improved method for hardening articles with irregular contours such as gears or racks, said method including pre-heating the gears with a relatively low frequency induction current, and thereupon contour hardening the article by the application of a substantially higher frequency electrical induced current, said combination being designed to eliminate tensile stress in the critical areas of the provide a production line heat treating processforirregular metallic objects. by use of which such objects fully treated shall be produced at a high time rate.

An ancillary object of our invention isto provide, an'irregular metallic object, such as a gear, having a substantially unimpaired hardened surface region produced at a: relative. low cost per item and having high resistance to repeated impacts.

More concisely stated, it is an object of our invention to provide a low-cost, high-speed method of heat-treatment, apparatus for Dracticing the method and articles produced by the practice of the method.

In accordance with our invention we provide a metallic object which has a thin hardened layer only in the region where major stresses are applied to it. Thus, we provide a gear which has a shallow, hardened contour casing. Gears for the transmission of standard automotive vehicles and small tractors, produced in accordance with our invention, have a hardened contour of average thickness of the order of .030 inch. If a section of such a gear by a plane perpendicular to its axis is observed, the hardened area will be seen to be a band which extends along the contour of the teeth narrowing towards the root. The inner boundary of the band is a curve spaced approximately .050 inch from the top of the tooth and .030 inch from the root.

Certain gears of a transmission are moved axially to engage with other gears. During this operation the sides of the teeth of one of the engaging gears impacts against the sides of the teeth of the other. To reduce the shock engagement (the clash) the engaging sides (the clashing faces) of the teeth have rounded surfaces. It is desirable that these surfaces be hardened.

In accordance with our invention the clashing surfaces are hardened to a distance just below the region of engagement of the teeth. In practice this hardening customarily extends to a distance approximately half-way between the pitch line and the root. For gears included in the transmissions of standard automotive vehicles and tractors, the thickness of this layer as measured'from'the side surface of the tooth varies from approximately .050 inch at the point nearest the tip of the tooth to zero where the hardened area disappears below the pitch line.

Surface wear of teeth, according to our invention by reason of impact during gear shifting and rotation is minimized by the band surface.

In accordance with a further aspect of our invention, we provide a novel method for producing gears of this type. A gear to be heat treated in accordance with our invention is first pre-heated inductively to a substantial depth below the roots of the teeth (in the case of a small gear throughout). This object is accomplished by subjecting the gear to the electromagnetic field produced by a generator of audio frequency oscillations having a frequency preferably lying between 960 and 15,000 cycles per second, Each gear is maintained in the field until the temperature of the heated region attains a magnitude of the order of 900 F. Thereafter, the gears aresubjected to a radio frequency electromagnetic field. The frequency of the oscillations which produce this field preferably lies between 90,000 and 450.000 cycles per second. The teeth and roots of the gears are heated by the radio frequency field, to the desired depth, to the heat treating temperature (approximately 1600 F.). Thereafter, the teeth are quenched by projecting on them a liquid such as water or brine or imm'ersing them in a submerged oil spray. The generator which produces the radio frequency field preferably has sufficient power to provide energy at the rate of approximately 6% kilowatts per square inch to the surface area treated. Since it is desirable that the hardened Sk n. be

- netic field of different frequencies.

produced only over the surface of the teeth, the total surface area to be heated is in effect the surface area of the teeth. This area is approximately 21r pitch diameterxtooth face width. The teeth of the gears are thus heated to the hardening temperature, only in the thin outer layer where the hardened material is to be produced. The depth of the layer heated may be increased Within narrow limits by decreasing the power per square inch of surface treated and increasing the time interval during which the high frequency power is impressed.

Excessive stresses are avoided during the prequenching heating operation because the differential in temperature between the regions to be hardened and the other regions of the gear is relatively small (approximately 900 F.). These stresses and the higher stresses resulting from metallurgical changes following the quenching operation are moreover absorbed by the resilient cushion provided in the unhardened portion of the teeth.

The method of heating in accordance with our invention involves the application of electromag- In its more limited aspects our invention involves the steps of subjecting the articles to be treated first to an audio frequency field and subsequently to a radio frequency field. Our invention in its broader aspects includes within its scope the steps of subjecting the articles to be treated to fields of different frequencies of any classification. For example, two audio frequencies, 60 cycles per second and 9600 cycles per second, may be applied. 0n other occasions one or both of the fields may be of radio frequency, of ultra-high frequency or of microwave frequency. For example, fields having frequencies of the order of 100,000 cycles per second may be impressed for pre-heating purposes and fields of a frequency of the order of 1 to megacycles (or even 10,000 megacycles) may be impressed for the pre-quenching heating.

The frequencies of the pre-heating and prequenching fields depend on the dimensions of the objects to be treated, the area to be pre-heated, the area to be hardened and the depth of the hardened material. The larger the object to be heated the lower the pre-heating frequency. The pre-quenching frequency selected will depend on the contour to be hardened (the pitch of the gear to be hardened) and on the minimum penetration desired. A pre-quenching field of a given frequency will produce a predetermined minimum penetration. The penetration may be increased by decreasing the power in the field and increasing the time during which the field is applied. To decrease the penetration below the minimum the frequency of the field should be increased substantially.

The method of heat treatment in accordance with our invention should be distinguished from multiple frequency step heating, such as is described, for example, in the patent to Seede 1,646,498. Broadly our invention involves multiple frequency heating as a part of a hardening process and such heating in any hardening process is within the broad scope of our invention. However, mere subjecting of an article to be treated to two frequencies, one a low frequency and the other a high frequency, will not produce a hardened article in accordance with the more specific aspects of our invention. The power applied durin the pre-heating operation must be such as to produce a satisfactory differential between the pre-quenching and pro-heating tem peratures. The power applied during the prequenching operation must be of the proper magnitude to heat the region which is to be hardened. If an article'is subjected to fields of different frequencies without consideration of the power impressed, it will be irregularly heated and when subsequently quenched, the desired hardened surface will not result. If the pre-heating temperature is toolow, excessive stresses may develop during the ore-quenching heating. If this The novel features that we consider characteristic of our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which:

Figure 1 is a view in section, by a plane perpendicular to the axis, of a portion of a gear in accordance with our invention;

Figure 2 is a view in section through line 22 of a gear of the type shown in Fig. 1 which has a clashing surface;

Figure 3 is a view partly in top elevation and partly diagrammatic showing conveyor apparatus for producing hardened metallic objects in accordance with our invention at a high time rate;

Figure 4 is a view in side elevation of a portion of the apparatus shown in Figure 3;

Figure 5 is a view in section taken along line 55 of Figure 3.

The gear shown in Fig. 1 has a shallow hardened region H] which extends only along the contour of the teeth I I and roots l2. The hardened contour is widest at the top and narrows toward the root of the gear. If the gear is of the type having flat side surfaces, the hardened contour along the side surface has the same form as in the section produced by a plane perpendicular to the axis of the gear (as shown in Fig. l). The hardening does not extend over the side surfaces of the teeth beyond the depth to which it extends throughout the sections.

Gears included in a transmission which are moved during the gear shift operation have side surfaces designed to minimize the shock during the engaging operation. We shall designate such surfaces as clashing surfaces. of this type is shown in section in Fig. 2. The clashing surface 15 is curved to facilitate the engagement of a gear with other cooperative gears of the transmission system. In accordance with our invention, the hardened zone [5 of such a gear extends along the clashing surface to a distance approximately one-half way between thepitch line I? and the root l2.

The hardened band along the contour of the teeth and roots and over the clashing surface is A tooth of a gearnarrow compared to the length of the teeth. In gears of the type included in the standard auto-. motive vehicles and tractors, the depth of the hardened band at the top of a tooth is of the order of .050 inch. This depth at the root is of the order of .030 inch. The band along the-clash:

ing contour narrows from approximately .050:

inch to zero.

Figures 3 to 7 show a conveyor system for; producing gears in accordance with the method; of

this invention.

The gears or work to be treated are indicated by the reference character 20. The gears 20 are carried by a plurality of work holding carriers 2|. As best shown in Figs. 6 and 7-, each carrier- 2! comprises a bottom plate 22 having a cen-v trally located opening 23. Radially disposed-lo-.

eating members 24 are provided to place the axis of the gears in axial alignment with the opening;-

23. A hollow quench ring 25 is positioned on the carrier 2!. The quench ring is provided with internal openings 26 through which a quenchingmedium may be distributed. A portion?! of the carrier 2| is in sliding engagement with a longitudinal support 28. The longitudinal support 28v is provided with a quench fluid conduit 29 atv each of the quench stations as will presently be more fully described. A guide pin 30 extends;

downwardly from each carrier 2|. The guide pin 3 is adapted to slide between guide rails 3|".

The carriers are connected to a chain 3| as indicated at 38 of Figure 6. The chain 3| is trained about sprockets 32, one of which is positioned. at

each end of the apparatus as shown in Figure: 3.

The conveyor chain 3| is indexed and advanced intermittently by operation of a dog 32." which engages the same. The dog 32" is actuated by an indexing air cylinder 33 which reciprocates' a piston arm 34. As the chain is indexed intermittently, the carriers and the gears supported thereon are advanced through the various sta-r. tions of the conveyor system.

The stations at which the gears 20 are sub.

jected to treatment are longitudinally disposed along the line of the conveyor system. The preheat stations are designated from I to 4, inclusive. Station 5 is the heat and pro-quench sta-. tion. Station 6 is a quench station, and-1, 8,,and 9 refer to the reheat stations.

Stations I to 4 are provided with electrical induction coils or blocks 48 which, are energized. from a suitable audio frequency generator 41.

Station 5 is provided with an electrical induction block or coil 42 which is energized from a radio frequency generator 43. Station 6 is the quench station and does not have an induction block. Stations, 1' to 9V inclusive are provided with inductor blocks 5 which are energized from themotor generator 4! The circuits leading to the A. F: and R. F. generator are shown diagram.- matically in Figure 3.

Under each of the 9 stations, and in axial alignment with the openings 23 in each carrier, are a plurality of spindles 45. The spindles are mounted for rotation in suitable bearings 46 which in turn are carried by a supporting frame 41. The supporting frame 41 includes vertical spaced end members 48. Vertical members 49 slidably engage the end members 48, and have secured thereto longitudinal supports 50, towhich the bearings are attached. The spindles 45 are rotated by means of a driving chain 51 which engages sprockets 52 which are connected to the spindles 45. A suitable motor 53 drives the chain 5| through a driving connection 54.

As best shown in Figures 4 and 5, air cylinders 55 are provided. The air cylinders 55 serve to move the supporting members 50 and the spindles 45 as one unit upwardly and downwardly, the vertical members 49 slidably engaging the vertical end members 48. An air conduit 56 is suitably connected to a tank 51 containing a supply of compressed air.

An ejector cylinder 60 is provided, as indicated in Figures 3 and 5. The ejector cylinder 60 is actuated by compressed air from a conduit 6| in communication with the source of compressed air. The ejector cylinder 60 is positioned adjacent' to an ejection station 62 which includes a raising cylinder 63, hydraulically synchronized with the cylinder 60. As the work carrier arrives at the ejector station the gear is'lifted upwardly from the carrier by means of the raising cylinder. The ejector cylinder thereupon pushes the gear into a hopper 64.

A plurality of valves 10 control the flow of air to the ejector cylinder 50, the indexing cylinder 33, and the air cylinder 55. The valves Ill are opened and closed by means of the operation of corresponding cams H rotated by means of a motor T2. A cam operated switch 13 also controls the closing and opening of the generator circuits to control the supply and interruption of the audio and radio frequency power. The quenching is also controlled from a cam 15 which operates a switch 16 and valve TI. The valve TI is connected to conduits 18 which in turn are provided to supply a quenching medium to the quench stations 5 and 6. The cam 15 operates the R. F. circuit and also the quench supply valve. The cams are of such contour that the operation and-timing of the valves and switches is properly synchronized.

The inductor blocks 40, 42, and M are shown as standard type rings for heat treating various types of gears. As shown in Figure 2 the inductor blocks may also be provided with a projecting portion 80 for suitably forming the hardened surface band of the clashing type gear shown in this figure. Different shapes of inductor blocks can be provided depending on the type of gear that is intended to be hardened.

During the operation the work carriers 2| are indexed adjacent to a loading table 90. At this point the operator places a gear 20 upon each of the carriers. By virtue of the cylinder 33 and dog 32", the carriers are indexed to the various heating and quenching stations. The carriers are brought to rest at the various stations with the openings 23 and the gears in axial alignment with the corresponding spindles 35. At this point all of the carriers and the gears are situated below the inductor blocks and have to be moved upwardly into these blocks for the heating operation. The spindles 45 are thereupon moved upwardly as a unit due to the operation of the air cylinders 55. As the spindles 45 move upwardly they extend through the openings 23 in the work holders; and continued upward movement lifts the gears 20 free of the work holders until the gears are positioned within the inductor blocks of the various stations. After a predetermined time interval, the treating for all of the gears is completed, and the spindles are moved downwardly by operation of the cylinders 55, and the gears are thereupon again deposited on the work holders 2|. The work holders are then again indexed to the succeeding station where the above described process is repeated.

As shown in thediagrams in Figures 3 to 5, the gears are subjected to a number of hardening and quenching treatments. A gear to be properly treated is first indexed through the four pre-heating stations I, 2, 3, and 4. At each of I these four stations the gear is subjected to an electromagnetic field of audio frequency, preferably between the limits of 960 to 9600 cycles. The field at each station is produced by the inductor blocks 40 which are energized from the audio frequency generators M. The time interval to which the gear is subjected to the field is so related to the time it is indexed that after being heated at the fourth station the gear has a temperature near the order of 900 F. This temperature is substantially below the transformation temperature for steel.

After the gear has been indexed to the fourth station it then passes onto the fifth, where a heat and quench operation is effected. At this station the contour hardening of the gear takes place. The inductor block 42 is energized from a radio frequency generator 43 preferably having a frequency between 90,000 and 450,000 cycles. Accordingly, the gear at this station is subjected to a radio frequency electromagnetic field. In view of this high frequency, the gear is heated only over a shallow band extending along the contour of the teeth and roots. The depth of the band is determined by the frequency of the field and within limits by the duration of the interval during which the radio frequency field is applied. The duration of the radio frequency heating is substantially shorter than the duration of preheating. The power output of the radio frequency generator is such that the radio frequency energy supplied to the gear is just sufiicient to produce the desired pre-quenching temperature along the contour. For a gear of the type included in the transmission of standard automotive vehicles, pre-quenching power of the order of 6.5 kw. per square inch of treated surface is impressed for a time interval of one second. As best shown in Figure 5, after the gear has been subjected to the radio frequency field, it is then lowered into thequench coil 25. At this time interval a quenching fluid (preferably of water or oil) is projected on the gear 20. The quenching fluid (preferably water) is supplied through the conduit 29 and into the quench ring 25 whereupon it is distributed through the openings 26 and projected on the gear. In the practice of our invention the delay between the interruption of the radio frequency field and the application of the quenching fiuid may vary from 0 to .5 second for gears used in the transmission of standard automotive vehicles and small tractors.

The gear is then indexed to the sixth station where again a quenching medium is supplied. As the gear is moving from the fifth to the sixth station, the quenching operation continues. When the gear reaches this station the contour hardening has already been eifected and the suitable quench therefor has been provided. At the sixth station, however, the gear is cooled, not

only at its periphery but throughout its whole. body. The duration of the quenching immedi-' ately following the treatment at station 5 should the pre-heating fields and operate to reheat the sea s to approximately 350 It is believed that the "gears are drawn during this operation. A gear, after leaving the ninth station, is moved "adja'centto'the ejector station 62. At 'this'point the raising cylinder 63 raises upwardly to remove the'gear from the carrier 2!. The ejector cylinder '80 thereupon operates to push the gear into "the hopper l0.

During any complete operation the indexing cylinder 33 is first supplied with air to index and move the conveyor. The spindles thereupon are raised by means of the cylinders 5'5 to which compressed air is supplied and the gears are thereby raised into the treating "media. The audio and radio frequency circuits are then closed and energy is "supplied to the gears. While the pre-heating and final-quenching (at station 6) continues the radio frequency energy is interposed and the gear 2!! at the pre-quenching station 5 is-quenched. While the gears in the treating line are subjected tothe treatment the ejector cylinder ejects the treated "gear into the hopper.

"Our invention is'not to be limited to the specific treating line shown in Figs. 3 to 7 In the prac tice of certain aspects of our invention we may dispense with the final quenching operation and the drawing operations. In other aspects of our invention we may dispense [with the drawing operation, retaining the final quenchin operation or with the final quenching operation, retaining the drawing'operation. The duration of the treatment at the various stations may also be varied over a wide range. In treatin gears included in transmission of standard automotive vehicles or tractors, in accordance with our invention the pie-heating fields and the drawing fields are preferably'applied for 4 second periods, for indexing at intervals of 2 seconds. The radio frequency field is applied to these gears for .4 of a second to 1 second. The preliminary quenching operation has a duration of the order of :3 seconds. The quenching fluid may be projected on the gears just before the radio frequency field is interrupted. Alternatively a delay of as'long as 25 secondmay be introduced between the interruption of the radio frequency field and projection of the fluid. Excellent results have been-obtained-by introducing adelay of .2 second between the pre-quench heating and the'quenching operation.

In discussing the apparatus shown in Figures 3 to 5, we have restricted ourselves to the treatment of gears. Other metallic objects such as crank shafts, racks, ratehets and the like may be treated with this conveyor equipment.

In practicing our invention We have successfully treated gears shaving the following dimensions:

Outside diameter inches 2%; to 7 Pitch 7 to 8 Width inches to The gears treated were composed of carbon steel having a carbon content of .48% to 53% and a manganese content of .80% to 1.0%. These gears were ,pre-heated by subjecting them to an electromagnetic field having a frequency of 9600 cycles produced by a generator, the power output of which was varied between 25 kilowatts and 100 kilowatts. The zones of the ,gears to be hardened were then subjected to radio frequencyfields '10 of duration 4 seconds at intervals of 2 seconds. (Satisfactory results can also be obtained by heating the gears "continuously during time intervals equivalent to that corresponding to the pulsation heating.) The RpF. was supplied from .4 to 1 second. The heat input at the radio frequency was at the rate of approximately 6 kilowatts per square inch for the total surface eated.

'While we have shown and described certain specific embodiments of our invention, many modifications thereof are possible. Our invention therefore 'notto be restricted, except as is necessitated by the prior'art and by the spirit of the appended claims.

What is claimed is:

Ap a atus for duct h t e n a e ab e m allic 'bJ'ect com r si a plura it of cheese ve r ea i e a i means for producing a alternating electromagnetic field of a fi1i t frequency at each of said stations, a -pre-fquejnc'h heating station adjacent said pre-heatingstations, means for producing an electromagnetic "field of substantially higher frequency than said first frequency at said prequench heating station, means for supplying a quenching fiuid at said pre-quench heating station and mea s for subjecting said object to the field and quenehingfiuid for substantially equal time intervals at each said stations and for 'moving said object from station to station during substantially equal time intervals.

'2. Apparatus for h at treating a hardenable metallic "object comprising a plurality 'of successive pre-heatingjsta'tions, means for producing an alternating electromagnetic field of audio freeill) jquency at each of said stations adjacent said 'pre-he'ati'ng stations, a pre.-.,quench heating sta 'tion, means for producing an electromagnetic held of radio fre uen at sa p e 'eneh h a in stat-11m m ans for supplying a qu nchi fluid at said preequench heating station and means for moving said object from station to station during substantially equal time intervals.

3. Apparatus for heat treating a hardenable metallic object comprising a plurality of successive pre-heating stations, means for producing an alternating electromagnetic field .of a first irequencyat each of saidstations, a preequench heating station following said p e-heating stations, means for producing an electromagnetic field of substa ntially higher frquency than ssaid :fire ency at said er -qu n h -heat ns t on means fo sup l ng a q en h ng i d a said preequench heating station, a plurality of draw ng s amen .fq producing an e ecr ma e ic fiel of a d firs ri ed-u n a each ai draw n st men :fQ l bi t s i e to $ILeI iIZ t=fI QU HYL l p odu means and to the other said field D Oducin means and quenching Iluid means for substantially equal time intervals at each said stations an ;:f.0.r movin s i ;o.bie. t from station to station during substantially .equal time intervals, isaidsobject passing in sequence through-said preheating stations, through said preequench heatingstationiand through said drawing station.

4. The mass production inethod-of'heat treating metallic articles which are hardenable by heating and quenching, said articles having an irregular peripheral surface, which comprises the steps of -pre-heating each of said articles with a relatively "-low trequency curren-t in a plurality of "discrete impulses "of substantially equal predetermined time durations at substantially equal of time sufficient said surfaceto be hardened to approximately predetermined time intervals, thereby heating each of said articles to a temperature of the order of 900 F., one of said impulses being applied at each of a plurality of preheating stations, and after a finite interval of time heating each of said articles with a higher frequency current for a time duration sufficient to reach a temperature such that a hardened contour zone of relatively shallow depth is obtained upon quenching each of said articles, and thereafter quenching each of Said articles.

5. The mass production method of heat treat- I ing metallic articles which are hardenable by heating and quenching, each of said articles cycles per second in a plurality of impulses at a plurality of pre-heating stations, one of said impulses at each of said stations over a period to bring the temperature of 900 F., and after a predetermined time interval heating each of said articles with an induced current of substantially higher frequency over a period of time to bring the temperature of said surface to be hardened to a level sufficient to develop a relatively shallow hardened zone upon quenching each of said articles in a coolant, and thereafter quenching each of said articles.

6. The mass production method of heat treating metallic objects which are hardenable by heating and quenching, each of said objects having an irregular peripheral surface comprising the steps of pre-heating each of said objects with a relatively low frequency induced current within the limits of 960 to 15,000 cycles per second in a plurality of successive substantially equal timed and spaced impulses until the contour of said objects reaches a temperature of the order of 900 F., one of said impulses being at each of a plurality of preheating stations, and after a predetermined interval of time heating said objects with a higher frequency induced current until the contour of said objects reaches a temperature above the critical hardening temperature of the metal, and thereafter quenching said objects.

7. The mass production method of contour hardening an irregular peripheral surface metallic object having one or more teeth projecting from the surface thereof, said object being hardenable by heating and quenching, which comprises heating a surface zone of said object with a relatively low frequency induced current in a plurality of equally spaced and timed discrete impulses, thereby bringing the root area of each tooth to a temperature of the order of 900 F., one of said impulses being applied at each of a plurality of preheating stations, and after a predetermined time interval heating said article with a substantially higher frequency induced current to bring primarily the tip area of each tooth to a temperature above the transformation point of the metal, and thereafter quenching said object whereby a hardened contour is effected on each tooth.

8. The method of heat treating an article of hardenable metal, said article being hardenable by heating and quenching and having an irregular peripheral surface, which comprises pre-heating the article by applying to it a plurality of short pulses of relatively low frequency induced current at spaced intervals, each of said impulses being applied at a different one of a plurality of successive stations, moving said article from each of said stations to the succeeding station,

the article being pre-heated' to reach a temperature of the order of 900 F. when the preheating operation in the last of said stations is completed, moving said article to a further heating station, and after a predetermined time interval heating said article with a relatively high frequency induced current to a temperature above the transformation point of the metal such that a hardened contour zone of relatively shallow depth is obtained upon quenching said article in a coolant, and thereafter quenching said article.

9. The method of heat treating an object of hardenable metal, said object being hardenable by heating and quenching and having an irregular peripheral surface which comprises the steps of heating the object by applying to it short pulses of relatively low frequency induced current at spaced intervals, moving said object through a series of successive heating stations, one of said pulses being applied at each of said stations until said object reaches a temperature of the order of 900 F., moving said object to a further heating station, and heating said object with a relatively high frequency induced current to a temperature above the transformation point of the metal such that a hardened contour zone of relatively shallow depth is obtained upon quenching said object with a coolant, and thereafter quenching said object.

10. The mass production method for heat treating hardenable metallic objects, each of said objects being hardenable by heating and quenching and having an irregular peripheral surface, comprising the steps of pre-heating each of said objects by applying a plurality of short impulses of relatively low frequency induced current at spaced intervals, moving each of said objects through a plurality of successive heating stations, one of said impulses being applied at each of said stations until the contour zone of each of said objects reaches a temperature of approximately 900 F., moving each of said objects to a further heating station, and after a predetermined time interval heating each of said objects with a substantially higher frequency induced current for a time duration such that the contour zone of each of said objects reaches a temperature above the critical hardening temperature of the metal, and thereafter quenching each of said objects.

HOMIER F. KINCAID. JOHN D. GRAHAM. ROBERT M. BAKER. EDWARD M. WHARFF. JOHN A. REDMOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,646,498 Seede Oct. 25, 1927 1,748,465 Summey Feb. 25, 1930 2,051,915 Sykes Aug. 25, 1936 2,167,798 Denneen et al Aug. 1, 1939 2,170,130 Denneen et al Aug.- 22, 1939 2,202,758 Denneen et a1 May 28, 1940 2,202,759 Denneen et al May 28, 1940 2,209,303 Somes July 23, 1940 2,238,082 Somes Apr. 15, 1941 (Other references on following page) Number Name Date UNITED STATESPATEN IS 2,414,362 Denneen et a1. Jan. 14, 194': Number Name Date 2,444,257 Jordan June 29, 1948 2,279,216 Way Apr. 7, 1942 2,280,064 Denneen et a1 Apr. 21, 1942 5 FOREIGN PATENTS 2,351,439 Livingston June 13, 1944 Number Country t 2,364,623 Denneen et a1 Dec. 12, 1944 434,31 Great Britain May 4, 1 3 2,397,990 Sherman Apr. 9, 1946 2,400,472 Strickland May 14,1946 OTHER REFERENCES 2,408,229 Roberds Sept. 24, 1946 10 Metal Progress, July 1943, pp. 78-83. 

4. THE MASS PRODUCTION METHOD OF HEAT TREATING METALLIC ARTICLES WHICH ARE HARDENABLE BY HEATING AND QUENCHING, SAID ARTICLES HAVING AN IRREGULAR PERIPHERA SURFACE, WHICH COMPRISES THE STEPS OF PRE-HEATING EACH OF SAID ARTICLES WITH A RELATIVELY LOW FREQUENCY CURRENT IN A PLURALITY OF DISCRETE IMPLUSES OF SUBSTANTIALLY EQUAL PREDETERMINED TIME DURATIONS AT SUBSTANTIALLY EQUAL PREDETERMINED TIME INTERVALS, THEREBY HEATING EACH OF SAID ARTICLES TO A TEMPERATURE OF THE ORDER 